1. Field of the Invention
This invention relates to the art of sensing defects in workpieces and, more particularly, to sensing such defects in thin wall, light metal castings.
2. Related Art Statement
Non-destructive tests are those tests which detect factors related to the serviceability or quality of a part or material without limiting its usefulness. Material defects such as surface cracks, laps, pits, internal inclusions, bursts, shrinking, seams, hot tears, and composition gradients may be detectable, depending on the material and method selected.
Radiation techniques are commonly employed to detect internal discontinuities. Such techniques include, for example, the use of X-rays, or gamma radiation. Gamma radiation is more suitable for field applications than X-ray techniques, since less complex equipment is required. In both methods, radiant energy is passed through the object being examined and is allowed to impinge upon sensitive film. The dark areas on this film are formed by internal defects which absorb less radiation than the sound metal. In general, X-rays are preferred over gamma radiation for laboratory testing since they offer greater control over intensity, and produce sharper pictures.
Inspection and evaluation of the semi-finished product is often necessary. But because semi-finished products may be hot, an observer cannot examine them conveniently. Most approaches presently in use therefore call for forming an image of the weak points or defects at a distance. Such methods may differ both in the means of obtaining the image and in the means of recording this image and using the recording to repair the semi-finished product.
These processes usually require complex equipment and expensive installation. Sometimes, it may be difficult for an observer to distinguish between those points on an image which actually correspond to the state of the semi-finished product and those points resulting from disturbance of the transmission signals issuing from those features. Accordingly, the recording of such images--frequently of low resolution--and the use of such images by the repairer can lead to non-uniform semi-finished products.
Inspection of thin wall light metal castings is presently performed by real time fluoroscopy (X-ray). Fluoroscopic equipment is relatively expensive, requires isolation from employees, occupies significant floor space, requires multiple safety interlocks, consumes a large amount of power, and requires special operator training with federal and state (local in some instances) licensing. Nevertheless, typical fluoroscopic (X-ray) analog image techniques may be partially effective in detecting certain defects. However, if the workpiece has various thicknesses such that its cross-section is non-uniform, multiple images of varying X-ray intensities must be made to detect all defects.
The use of a camera and infrared television to give images of defects is reported in U.S. Pat. No. 4,671,474 at col. 2, lines 6-8. Such techniques produce results which are said to be disturbed by the presence of powders, oxidation spots, and surface irregularities, besides providing little indication of the depth of the defect.
Accordingly, the relatively low resolution of a standard infrared system would be unable to detect small internal defects.
Faced with problems posed by conventional techniques, it would be desirable to have access to a relatively simple process and means for defect detection which produces an image which faithfully portrays defects within a workpiece.
Additionally, it would be desirable to be able to utilize an enhanced, high resolution image which may, for example, be enhanced digitally.
Thus, it would be desirable to be able to use a system, such as an infrared thermographic system for considerably less cost than is associated with, for example, conventional fluoroscopic equipment. Ideally, such a system would not take up an excessive amount of floor space, nor consume excessive power. Finally, it would be useful to be able to position a monitor and controller remotely from the workpiece by the use of suitable cabling. In such a configuration, safety concerns are minimized because such equipment would detect defects by emitted temperature (infrared) radiation with minimal training of the operator.